This invention relates to a material for in-tube components and a method of manufacturing it, capable of manufacturing with good formability in-tube components such as shadow masks, frames, inner shields and bimetallic elements used in electronic tubes such as color cathode ray tubes.
So-called in-tube components of colour cathode ray tubes, such as shadow masks, frames, inner shields and bimetallic elements, have previously been made of materials such as rimmed steel or Al killed steel, which has good etching characteristics and formability and a surface on which it is easy to form an oxide film that contributes to lessening reflection of the electron beam. However, as further improvements in colour cathode ray tube quality, i.e. what is known as "easy of viewing" or "high definition" of the displayed image have come to be demanded, to deal with the requirements of various new media, drawbacks have appeared to the use of rimmed steel or Al killed steel as referred to above for shadow masks, frames, inner shields and bimetallic elements.
Specifically, when the colour cathode ray tube is operated, the temperature of the aforementioned members rises to 30.degree.-100.degree. C., causing for example what is called "doming", due to strain in the formed shape of the shadow mask produced by its thermal expansion. This results in misalignment of the relative positions of the shadow mask and the phosphor surface, producing what is known as "purity drift" (PD) of the colours. Particularly in the case of high-quality colour cathode ray tubes, the apertures, and the aperture pitch, of the aforementioned shadow mask are very small, so that the proportion of relative misalignment becomes large, preventing the use of in-tube components made of the aforementioned rimmed steel or Al killed steel. The above problem is particularly marked in the case of colour cathode ray tubes of high curvature with reduced image distortion and reflection of external light.
Previously, therefore, Ni-Fe alloys of small thermal expansion coefficient, such as invar (36Ni-Fe) or superinvar (32Ni-5Co-Fe) were used as the material for forming in-tube components of this type, for example as proposed in U.S. Pat. No. 4420366 (Oka et al.), Japanese Publication No. Sho. 42-25446. Japanese Patent Laid-open No. Sho 50-58977, or Japanese Patent Laid-open No. Sho. 50-68650.
However, Ni-Fe alloys of this type have poor thermal conductivity. Not only does this make them liable to accumulate heat, but also makes them liable to what is known as "spring-back" (depressions towards the electron gun, from the normal spherical surface of the shadow mask). This also gives problems, in etching characteristics and formability such as irregular aperture diameter being obtained when the apertures of the shadow masks are formed by etching.
In more detail, this spring-back exhibits the correlation shown in FIG. 1 with for example the 0.2% yield point of the material. Thus, the lower the 0.2% yield point, the smaller the amount of spring-back and the better the formability.
Previously therefore, to lower the 02.2% yield point of the material, trials were made of subjecting it to vacuum annealing at over 1000.degree. C., or of forming working of the intube component in the temperature range 100.degree.-200.degree. C. Various methods of improving the etching characteristics were also tried. However, the situation is that none of these methods resulted in etching characteristics of formability being attained which were as good as those of the rimmed steel or Al killed steel referred to above. This therefore imposed a limit on the extent to which the quality of colour cathode ray tubes could be improved.